26 Transformationtoughened Zirconia

Transformation-toughened zirconium oxide (TTZ) is another family of important high-strength, high-toughness ceramics that have been developed during the last 20-25 years [13, 14]. TTZ materials have fracture toughness values ranging from about 6-15 MPa ■ m1/2, compared to conventional ceramics with fracture toughness of about 2-3 MPa ■ m1/2. The mechanism of toughening in TTZ materials involves a volume increase due to a polymorphic transformation that is triggered when an applied stress causes a crack to form in the TTZ [15]. The volume increase only occurs for material adjacent to the crack and presses against the crack to keep it from propagating through the TTZ. Some forms of steel have a similar mechanism of toughening, so TTZ has sometimes been called ceramic steel.

Figure 2.7 shows the microstructure of one type of TTZ called partially stabilized zirconia (PSZ). It consists of lenticular-shaped precipitates of the tetragonal form of zirconia distributed throughout larger grains of the cubic phase of zirconia. The tetragonal grains are the ones that transform adjacent to a crack. Another TTZ is made up completely of tiny grains of the tetragonal phase and is referred to as tetragonal zirconia polycrystal (TZP). Both types are mentioned because they each have different properties, and one may be preferable for a specific application.

Transformation toughening was a breakthrough in achieving high-strength, high-toughness ceramic materials. For the first time in history a ceramic material was now available with an internal mechanism for actually inhibiting crack propagation. A crack in a normal ceramic travels all the way through the ceramic with little inhibition, resulting in immediate fracture. TTZ has fracture toughness (resistance to crack propagation) three to six times higher than normal zirconia

FIGURE 2.7 Microstructure of PSZ type of transformation-toughened zirconia. (Photo courtesy of Professor Arthur Heuer, Case Western Reserve University, Cleveland, OH).

TABLE 2.5 Successful Applications of Transformation-Toughened Zirconia

Golf cleats, putters, drivers and most other ceramics. It is tougher than cast-iron and comparable in toughness to some compositions of WC-Co cermet.

Table 2.5 lists some of the applications where TTZ has been successful. TTZ ceramics typically cost around four times as much as steel and two times as much as WC-Co for a part such as an extrusion die. In spite of the higher cost, though, TTZ often can provide sufficient increased life to justify its use on a life-cycle cost basis. The suppliers can provide information on life-cycle cost for existing applications and can probably estimate for similar applications.

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